1. Field of the Invention
The present invention relates to a lithium-ion secondary battery.
2. Description of the Related Art
The lithium-ion secondary battery is light in weight and has a high capacity compared to a nickel cadmium battery, a nickel hydrogen battery, and thus has a wide range of application for a power supply of a portable electronic apparatus. In addition, the lithium-ion secondary battery is a strong candidate as a power supply that is mounted for a hybrid vehicle or an electric vehicle. In addition, accompanying miniaturization and higher functionality of recent portable electronic apparatuses, high-capacity is expected for the lithium-ion secondary battery that serves as a power supply of these portable electronic apparatuses.
The capacity of the lithium-ion secondary battery mainly depends on an active material of an electrode. In general, graphite is used for a negative electrode active material. However, a theoretical capacity of graphite is 372 mAh/g, and a capacity of approximately 350 mAh/g has been already used in batteries that have been put into practical use. Therefore, it is necessary to realize new high-capacity in order to obtain a nonaqueous electrolytic solution secondary battery having a sufficient capacity as an energy source of high-function portable apparatuses in the future, and thus a negative electrode material having a theoretical capacity higher than that of graphite is necessary.
Therefore, currently, alloy-based negative electrode materials such as silicon and silicon oxide have attracted attention. Silicon is electrochemically intercalating and deintercalating lithium-ions, and is capable of realizing charging/discharging of a very large capacity compared to graphite. Particularly, it is known that the theoretical discharge capacity of silicon is 4,210 mAh/g, and shows a high-capacity 11 times that of graphite.
However, since these alloy-based material forms a lithium-silicon alloy during intercalation of lithium, and is changed from an original crystal structure, very large volume expansion is accompanied.
Particularly, when silicon is charged up to the maximum capacity, the volume theoretically expands 4.1 times. Therefore, an active material is detached from a current collector, and thus electrical conduction disappears. As a result, a charging/discharging cycle characteristic significantly decreases (For example, Japanese Patent No. 2964732).
Furthermore, in the case of using silicon or silicon oxide as the negative electrode active material, accompanying expansion and shrinkage of the negative electrode active material during charging/discharging, a negative electrode itself also expands and shrinks, and friction occurs at an interface between a separator and the negative electrode. Therefore, the negative electrode active material or a conductive auxiliary agent is peeled, and thus the charging/discharging cycle characteristic decreases.